Analog recording systems, for example, magnetic hard drives, are used in a wide variety of commercial products, such as digital video recorders, laser printers, Voice-Over-IP devices, high definition televisions, vehicle control systems, cellular phones, storage systems (e.g., redundant arrays of independent disks, or RAIDs), desktop and laptop computers, portable audio players, personal data assistants, and digital cameras. In addition to magnetic hard drives, other analog recording systems, such as optical and magneto-optical drives, are similarly used in numerous commercial applications. Commercial products are growing more complex, and as they do so, there continues to be a growing requirement for greater storage capacities. For example, the storage capacity of commercially available magnetic hard drives for desktop computers has grown from 2 gigabytes to over 200 gigabytes in the last decade.
One method of increasing storage capacity in a recording system while maintaining substantially the same or smaller form factor includes increasing the density of the storage medium. In magnetic hard drives, the density may be increased by reducing the magnetic grain size and/or the bit size. However, in some conventional longitudinal designs (where the magnetic grains and/or bits are horizontally aligned), the storage medium densities are approaching the super-paramagnetic limit (i.e., when the energy required to change the magnetic moment of a magnetic grain approaches the ambient thermal energy). To overcome such challenges, some manufacturers are beginning to use perpendicular designs, where the magnetic grain and/or bits are vertically aligned.
While perpendicular designs can achieve greater storage medium densities, they may also require additional circuitry in the read channel. Referring now to FIG. 1, a conventional read channel of an analog recording system may comprise an AC-coupling circuit 10, an amplifier 60, and an analog-to-digital converter (ADC) 90. In some analog recording systems, the AC-coupling circuit is the first element in the analog front end of the read channel and is connected to an element which reads data from the storage medium (e.g., a magnetic read head or a read head pre-amp). Generally, AC-coupling circuits are used to minimize voltage offsets between two or more components. For example, AC-coupling circuit 10 may reduce an offset between the read head pre-amp and the read channel circuitry.
However, and in contrast to longitudinal designs where the output of a read head corresponds to magnetic field transitions on the recording medium, a read head output in a perpendicular design corresponds to the polarization of the magnetic field. As a result, perpendicular designs have the potential of generating a constant, or nearly constant, signal output when the recording medium is uniformly magnetized. Such a constant signal output may result in the baseline (or median) output of the AC-coupling circuit drifting (or wandering) over time, which leads to a voltage offset between the read head and the read channel.
If not corrected, this baseline offset may cause amplifier 60 or ADC 90 to become saturated or otherwise operate outside an optimal parametric range, which may result in an erroneous data reading. One conventional approach to adjusting the baseline of the signal is to include a voltage shifter 20 in series with the AC-coupling circuit 10 and the ADC 90. However, such a series approach may introduce propagation delay through the analog front end of the read channel.
Therefore, it would be desirable to provide a system that is able to adjust the baseline of an analog signal in the analog front end of a read channel without introducing propagation delay.